In recent years, a flat-panel-ready self-emitting device has been receiving attention. The self-emitting device includes a plasma light-emitting display device, a field emission device, an electroluminescence (EL) device and the like.
Among them, in particular, an organic EL device has been demonstrated by T. W. Tang et al. in 1987 that high luminance emission can be obtained by a low voltage DC drive utilizing a structure in which the thin films made of a fluorescent metal-chelate complex and a diamine-based molecule are stacked, and its research and development have been pursued energetically. In these low-molecular-based organic EL devices, area-color type displays of green single color and of green color and additional colors such as blue and red have been commercialized, and the development for full-colorization is being activated at present.
As the organic EL device, there is a carrier injection type self-emitting device utilizing the emission that is produced when the electron and the hole that have reached a light-emitting layer are recombined. FIG. 1 shows a configuration of a typical organic EL device. A metal electrode is used for a cathode 11 and a transparent electrode is used for an anode 14 in order to take out the emitted light. An organic compound layer is sandwiched between these electrodes. In FIG. 1, the organic compound layer is composed of a light-emitting layer 12 and a hole-emitting layer 13.
Each of the organic layers that compose the organic compound layer generally has a thickness of approximately several tens of nanometers. As metal materials for the cathode, the metals having a small work function such as aluminum, an alloy of aluminum and lithium and an alloy of magnesium and silver are typically used. A transparent conductive material having a large work function such as indium tin oxide (ITO) is used for the anode.
The organic compound layer typically has a two-layer structure consisting of a light-emitting layer 12 and a hole-transporting layer 13 as shown in FIG. 1, or a three-layer structure consisting of an electron-transporting layer 22, a light-emitting layer 23 and a hole-transporting layer 24. The hole-transporting layer has the function for efficiently injecting holes from the anode into the light-emitting layer, and the electron-transporting layer has the function for efficiently injecting electrons from the cathode into the light-emitting layer. In addition, the hole-transporting layer and the electron-transporting layer have the (carrier blocking) function for blocking electrons and holes within the light-emitting layer, respectively, which is effective to enhance the emission efficiency.
A liquid crystal display that is already commercialized as a full-color flat panel display has achieved the full-colorization using color filters and the like. However, the organic EL device can self-emit primary colors of red, green and blue by appropriately selecting materials for composing the light-emitting layer and has excellent advantages of a higher speed response and a wider view angle than the liquid crystal display.
A dye-doped organic EL device in which a host material is doped with a fluorochrome is typically utilized because in the emission of each color of red, green and blue, it is difficult to obtain sufficient luminance and color purity by the light-emitting layer comprised of a single light-emitting material. This is the technique in which the material that composes a hole-transporting layer, an electron-transporting layer or a light-emitting layer in FIG. 1 or 2 is used for a host and doped with a very small amount of fluorochrome to take out the luminescence from the fluorochrome as a luminescent color. The advantage of this method is that the improvement of efficiency can be expected since the dye having a high fluorescent yield can be utilized and the selection of the luminescent color is greatly improved.
The light emission generally used in the organic EL device is taken out from fluorescence that is produced when the singlet excitons of the molecule in the luminescence center change to the ground state. On the other hand, the device that does not utilize the fluorescent emission through the singlet excitons but utilizes the phosphorescent emission through the triplet excitons is being investigated. Published exemplary references include, for example, the following Non-Patent Reference 1 and Non-Patent Reference 2.
The configuration in which the organic layer has four layers is mainly used in these references. It consists of a hole-transporting layer, a light-emitting layer, an exciton diffusion-preventing layer and an electron-transporting layer. The used materials are a carrier-transporting material and a phosphorescent-emitting material shown below.
An abbreviation of each material is shown below:    Alq3: aluminum-quinolinol complex    α-NPD: N4,N4′-Di-naphthalen-1-yl-N4,N4′-diphenyl-biphenyl-4,4′-diamine    CBP: 4,4′-N,N′-dicarbazole-biphenyl    BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline    PtOEP: platinum-octaethylporphyrin complex    Ir(ppy)3: iridium-phenylpyrimidine complex

In both of the above described non-patent references 1 and 2, high efficiency was obtained for the devices formed by using, as hosts, α-NPD for a hole-transporting layer, Alq3 for an electron-transporting layer, BCP for an exciton diffusion-preventing layer and CBP for a light-emitting layer and incorporating into them PtOEP or Ir(ppy)3 that is a phosphorescent-emitting material in a concentration of approximately 6%.
The phosphorescent-emitting material has particularly received attention because high light emission efficiency can be expected in principle. The excitons formed by the recombination of carriers consist of singlet excitons and triplet excitons, in which the probability of occurrence is 1:3. Conventional organic EL devices have been taking out the phosphorescence when the singlet excitons cause the transition to the ground state, in which the emission yield is 25% relative to the number of excitons formed in principle, which has been the upper limit in principle. However, if the phosphorescence from the excitons formed from the triplet is used, at least three times yield can be expected in principle. Furthermore, if the transition from the singlet to the triplet by the intersystem crossing that is high as energy is taken into account, the four times emission of 100% can be expected in principle.
Other references requiring the emission from the triplet disclose an organic EL device and a method for producing it (Patent Reference 1), a light-emitting material and an organic EL device using it (Patent Reference 2), an organic electroluminescent device (Patent Reference 3) and the like.
Furthermore, Non-Patent Reference 3 reports that in the above described EL devices, the amount of light that can be taken out to the outside changes according to the thickness of each of the functional films composing the devices, due to the light interference effect.
According to Non-Patent Reference 3, since there exists the optimum thickness of the electron-transporting layer relative to the emission wavelength, the thickness of each of the layers composing the EL device need to be optimized for each color in the EL panels having two emission colors or more. And in order to optimize it, Patent Reference 4 discloses a method for adjusting the thickness of the electron-transporting layer and optimizing the take-out efficiency of light.
Patent Reference 1: Japanese Patent Application Laid-Open No. 11-329739
Patent Reference 2: Japanese Patent Application Laid-Open No. 11-256148
Patent Reference 3: U.S. Pat. Nos. 5,698,858 and 5,756,224
Patent Reference 4: U.S. Pat. No. 6,541,130
Non-Patent Reference 1: D. F. O'Brien et al., “Improved energy transfer in electrophosphorescent device”, Applied Physics Letters (United States), 1999, Vol. 74, No. 3, p. 422
Non-Patent Reference 2: M. A. Baldo et al., “Very high-efficiency green organic light-emitting devices based on electrophosphorescence”, Applied Physics Letters (United States), 1999, Vol. 75, No. 1, p. 4
Non-Patent Reference 3: Yoshinori Fukuda et al., “An Organic LED display exhibiting pure RGB colors”, Synthetic Metals, 2000, 111-112, P. 1-6.
However, in order to achieve a device having a low price and high efficiency, it has been desired to prepare the device having high take-out efficiency of light using a process having smaller number of steps.